Performance Measurements of Generation and Transmission Systems
Performance Measurements of Generation and
Transmission Systems
Prepared By: Public Utilities Commission of Sri Lanka
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Performance Measurements of Generation and Transmission Systems
Content
Chapter 1 – Introduction
1
Chapter 2 – Transmission Performance Indicators
1
2.1 Transmission System Availability Indicators
1
2.2 Transmission System Power Quality Indicators
5
2.3 Supply Security Indicators
7
2.4 Transmission System Assets Utilization
8
2.5 Maintenance Cost Indicators
10
Chapter 3 – Generation Performance Indicators
11
3.1 Plant Factor
11
3.2 Running Plant Factor
11
3.3 Equivalent Full Load Hours
11
3.4 Generation Unit Cost
12
3.5 Auxiliary Consumption
12
3.6 Availability Factor
12
Chapter 4 – Special Performance Indicators for Renewable Power Plants
13
4.1 Wind Power Generation
13
4.2 Solar Power Generation
14
4.3 Biomass Power Generation
14
4.4 General
14
Chapter 5 – Conclusion
15
Chapter 6 – References
16
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Performance Measurements of Generation and Transmission Systems
1. Introduction
System and network performance standards are important issues in modern power system as they
ensure the efficient and secure functioning of the power system and appropriate quality of
electricity supply. Therefore there is a demand for common indicators that enable system operators
and others to evaluate their performance. These indicators measure the reliability of electric power
service and reflect operational problems. Moreover, they can also be used as a tool to compare or
benchmark the power system performance among utilities and generation plants.
This document is intended to identify the key performance indicators and their definitions which are
currently used worldwide to assess the technical and financial performance of generation and
transmission systems.
2. Transmission Performance Indicators
Electricity transmission companies normally measure their performance by using various types of
qualitative and quantitative assessments. They measure achievements of their objectives through
monitoring number of performance indicators. Performance Indicators are the parameters that are
related to transmission lines or transformers availability, service continuity or voltage wave. The
indicators for the present technical performance of the transmission system are useful when
planning the future developments to ensure a high degree of reliability of the transmission system.
2.1 Transmission System Availability Indicators
Availability of a Transmission System is expressed as a function of the Transmission Circuit Outages
or Interruptions and it is evaluated using indicators that measure the number of outages and their
durations. Availability Indices could be measured in terms of the scheduled (planned) and forced
outages separately or in term of overall outages. These interruptions also could be classified as
transient interruptions, long interruptions (Eg: More than 3 mins) and short interruption (Eg: Less
than 3 mins) according to their duration. Also the Transmission Line availability indices could be
measured categorized in terms of transmission voltage.
Transmission System availability of a Transmission system can be assessed through Individual
Performance Indicators and Overall Performance Indicators. Individual Performance Indicators are
used to measure Transmission System availability of each individual Transmission Line and each
individual Grid Substation Transformer.
The Individual Performance Indices are defined below.
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Performance Measurements of Generation and Transmission Systems
 Transmission Line Interruption Duration per Year (in Hours)
This measures the average time duration per year where a single transmission line is not
available in service.
∑
∑
Where;
UDL = Annual Unavailable Duration (in hours) of a Transmission Line
Hi,j = Duration of Outage “i”, that affected Transmission Line “j” (in hours)
NL = Total number of Transmission Lines
kt = Total number of Outages of Transmission Line “j” during the reported year
 Substation Transformer Interruption Duration per Year (in Hours)
This measures the average time duration per year where a single substation transformer is not
available in service.
∑
∑
Where;
UDT = Annual Unavailable Duration (in hours) of a Substation Transformer
Hi,j = Duration of Outage “i”, that affected Substation Transformer “j” (in hours)
NT = Total number of Substation Transformers
kt = Total number of Outages of Substation Transformer “j” during the reported year
 System Average Frequency of Outages of Transmission Line per Year
This measures the number of interruptions per year per transmission line of the system.
∑
Where;
SAFOL = System Average Frequency of Outages of Transmission Line per Year
NOj = Number of Outages of Transmission Line “j” during the reported year
NL = Total number of Transmission Lines
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Performance Measurements of Generation and Transmission Systems
Transmission Line outages also can be classified according to the duration of the outages (Eg:
Ranging from less than ten minutes to greater than 4 weeks) and total number of outages in each
time classification could be measured.
 System Average Frequency of Outages of Substation Transformer per Year
This measures the number of interruptions per year per substation transformer of the system.
∑
Where;
SAFOT = System Average Frequency of Outages of Substation Transformer per Year
NOj = Number of Outages of Transmission Transformer “j” during the reported year
NL = Total number of Transmission Transformers
Transformer outages also can be classified according to the duration of the outages (Eg: Ranging
from less than ten minutes to greater than 4 weeks) and total number of outages in each time
classification could be measured.
Overall System Performance Indices which are used to measure overall Transmission System
availability are defined below.
 Overall System Unavailability – Transmission Lines
This measures the average fraction of time (expressed in percent) that Transmission Lines are
unavailable and disconnected from the Transmission System.
∑
∑
Where;
SUL = System Unavailability (Transmission Lines)
Hi,j = Duration of Outage “i”, that affected Transmission Line “j” (in hours).
NL = Total number of Transmission Lines
kt = Total number of Outages of Transmission Line “j” during the reported year
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Performance Measurements of Generation and Transmission Systems
 Transmission Line Availability
SAL = (100 - SUL) %
Where;
SAL = Transmission Line Availability
SUL = System Unavailability (Transmission Lines)
 System Transmission Transformer Unavailability
This can be measured taking the fraction of time (expressed in percent) that Transmission
Transformers are unavailable and disconnected from the Transmission System or by considering
the extent of which the power transformer capacity remained unavailable.
∑
∑
or
∑
∑
Where;
SUT = System Unavailability (Substation Transformer)
Hi,j= Duration of Outage “i”, that affected Transmission Transformer “j” (in hours)
C = Capacity of Transmission Transformer “j” (in MVA)
j
NT = Total number of Transmission Transformers
kt = Total number of Outages of Transmission Transformer “j” during the reported year
TC = Total Installed Capacity of Substation Transformers (in MVA)
 Transmission Transformer Availability
SAT = (100 – SUT) %
Where;
SAT = Transmission Transformer Availability
SUL = System Unavailability (Substation Transformer)
 System Average Frequency of Outages per km
This measures the average number of Outages per km of Transmission Lines (Expressed in
number of outages per 100 km of lines)
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Performance Measurements of Generation and Transmission Systems
∑
∑
⁄
Where;
SAFO_L100 = System Average Frequency of Outages per 100km
NOj = Number of Outages of Transmission Line “j” during the reported year
NL = Total number of Transmission Lines
LONGj = Length of Transmission Line “j”
2.2 Transmission System Power Quality Indicators
To assess the performance of a transmission system not only the availability but also the quality of
the power must be considered. Power quality indicators shows how the transmission line
parameters comply with the defined standards.
The power quality measures and indicators are defined below.
 Frequency Excursions
Frequency variation is the deviation of frequency beyond a certain range from the nominal
supply frequency. Any frequency excursions outside these limits for a defined duration or more
(Eg: 60 Seconds) could be recorded as frequency limit violations.
In Sri Lanka the nominal fundamental frequency range shall be 50 Hz ± 1%. Frequency deviation
indices can be defined to find the number of time or duration that the system frequency goes
beyond the allowable range.
To figure out the extent of frequency excursions, frequency ranges can be also defined;
(If the nominal frequency is f)
1. Normal State
The Transmission System frequency is within the limit of ( f ± 1%)
2. Alert State
The Transmission System frequency is beyond the normal operating limit but within ( f ± 2%)
3. Emergency State
There is generation deficiency and frequency is below ( f - 2%)
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Performance Measurements of Generation and Transmission Systems
 Voltage Excursions
Voltage variation is the deviation of voltage in a certain range. Voltage deviations can be
identified by monitoring the bus bar voltages of the grid substations.
According to the defined Sri Lankan Transmission System Standards, bus bar voltage magnitudes
must comply with following allowed range of variation.
Voltage
220kV
132kV
Normal Condition
± 5%
± 10%
Single Contingency
-10% to +5%
± 10%
Voltage deviation indices can be defined to find the frequency or duration that the bus bar
voltages violate the allowed voltage range.

Number of voltage excursions exceeded n minutes per year

Percentage of time that the transmission voltage exceeds the permissible limits
 Voltage Unbalance
The phase voltages of a 3-phase supply should be equal in magnitude and phase angle. The
unbalance in phase voltages could be computed as follows;
% Voltage Unbalance = Max Deviation from Mean of {VRY, VYB, VBR} X 100
Mean of {VRY, VYB, VBR}
Where,
VRY = Voltage between R & Y phases
VYB = Voltage between Y & B phases
VBR= Voltage between B & R phases
Voltage imbalance indices can be defined as a limit of imbalance level (Eg: 220kV – 2% )
 Flicker
Voltage Flicker is a rapid change in voltage that is typically caused by user equipment that
distorts or interferes with the normal sinusoidal voltage waveform of the Transmission Network.
Flicker is the visual phenomenon which causes changes in the luminance of lamps and could be
annoying to people above a certain threshold.
In general the limits of the total Voltage Flicker at a Point of Common Coupling can be defined;
(a) ± 1% of the steady state voltage level, when these occur repetitively
(b) ± 3% of the steady state voltage level, when these occur infrequently
To assess the flicker occurrence, the indices can be defined to find the number of time or
duration that the flicker level goes beyond the allowable range.
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Performance Measurements of Generation and Transmission Systems
 Harmonic distortion
Harmonic Distortions are integer multiples of the fundamental power system frequency.
Harmonics of the supply voltage are caused mainly by the non-linear devices connected to the
system.
In general, the maximum total levels of harmonic distortion on the System under any operation
condition can be defined to not exceed the values;

220kV - a Total Harmonic Distortion of 2% with no individual harmonic greater than 1.5%

132kV - a Total Harmonic Distortion of 2% with no individual harmonic greater of 1.5%
To assess the harmonic distortions, the indices can be defined to find the number of time or
duration that the harmonic level goes beyond the allowable range.
 Transmission Losses
Transmission losses can be calculated as the difference between the total electrical energy
received from the generating plants and the total energy supplied to all bulk supply licensees. It
is usual to express losses as a percentage value rather than absolute value. The losses are
measured over a definite period, normally per year.
The definition of the transmission losses is defined as;
∑
∑
∑
Where;
EG = Total Energy Generated (MWh) during reported year
ET = Total Energy Sold to BSOB (MWh) during reported year
2.3 Supply Security Indicators
Power System security is the ability of the system to withstand sudden disturbances. To secure the
supply the Transmission system must be able to deliver the power even under abnormal or faulty
conditions.
 Energy Not Supplied (ENS)
This gives an estimation of the Energy not supplied to the connected Load due to the
Interruptions of transmission circuits over a year.
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Performance Measurements of Generation and Transmission Systems
∑
Where;
PDi = Power disconnected by transmission circuit Interruption “i” (in MW).
Hi = Duration of Interruption “i” (in hours)
kt = Total number of Interruptions during the reported year
 Overall Reliability of System
The Overall Reliability of Supply for a transmission system is calculated using the formula;
(
)
Where;
ENS = Estimated Unsupplied Energy due to unavailability of transmission circuits
 System Minutes Lost
This index measures the severity of each system disturbance relative to the size of the system, in
terms of duration of total system wide blackout. It is determined by calculating the ratio of
unsupplied energy during an outage to the energy that would be supplied during one minute, if
the supplied energy was at its peak value. One system minute indicates an equivalent of total
system interruption, with the magnitude of annual system peak, for one minute
The formula used for calculating the system minutes lost is;
When this index for a specific incident is greater than one minute, that incident can be normally
classified as a major interruption.
2.4 Transmission System Assets Utilization
Assets which directly involve in Transmission Supply system are very critical for the secure operation
are Transmission Lines and Grid Substation Transformers. Therefore they must be utilized
productively by keeping records of their loading capacities.
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Performance Measurements of Generation and Transmission Systems
 Grid Substation Overloading
Overloading of grid substations is defined based on the loading levels of grid substation power
transformers. Overloading of transformers must be avoided to avoid overheating, leading to
equipment damages and reducing the life time.
To assess the transformer overloadings, the indices can be defined to find the number of time or
duration that the Grid Substations have overloaded over the reported year.
 Transmission Transformer Utilization Factor
This measures the extent utilization of the transmission transformers with respect to their rated
capacities. It is the ratio of the maximum load on a transformer to its rated capacity.
∑
∑
Where;
UFPk = Transmission Transformer Utiliztion Factor
Pj = Recorded Peak Load of Transformer “j” (in MVA) during reported period
NT = Total number of Substation Transformers
Cj = Rated capacity of Transformer “j” (in MVA)
 Load Factor
Load Factor is an indicator of how steady an electrical load is over time. It is simply the average
load divided by the peak load in a system over a period of time. But normally load factor is
calculated subjected to the produced energy according to the following formula.
Load factor of any system must be tried to keep in its maximum by pulling down the
concentrated maximum demand and shifting the loads to periods of otherwise low usage. Load
factor maximization is essential in maintaining the security of supply of the countries in which
meeting the concentrated maximum demand is critical. Countries which have a flat load curve
own a higher load factor.
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Performance Measurements of Generation and Transmission Systems
2.5 Maintenance Cost Indicators
The Maintenance Cost incurred to operate a bulk supply operation business is also an indicator of
the financial performance of that operation. Maintenance cost includes the costs incurred that are
directly related to the well-being, security, integrity, reliability and availability of equipment and
installations. Technical labor/manpower costs, material cost, spares and vehicle costs associated
with normal or day today operation and maintenance activities as well as preventive and corrective
maintenance activities are also included.
 Transmission Lines Maintenance Cost Index (TLMCI)
This indicates the annual maintenance cost per unit length of transmission lines.
∑
Where;
TLMCI = Transmission Lines Maintenance Cost Index
NL = Total number of Transmission Lines
LONGj = Length of Transmission Line “j” in km
 Substation Maintenance Cost Index (SMCI)
This indicates the annual maintenance cost per Substation.
Where;
SMCI = Substation Maintenance Cost Index
NS = Total number of Grid Substations
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Performance Measurements of Generation and Transmission Systems
3. Generation Performance Indicators
Generation Performance Indicators are the parameters that are related to the availability, service
continuity or environmental impact of a generation plant or unit. Development of common
performance indicators is the first step in a process of creating large databases which would enable
power plant operators to compare their own plant performances with others, and make
improvements.
3.1 Plant Factor
The plant factor of a power plant is the ratio of the actual energy output of the power plant over a
period of time to its potential output if it had operated at full nameplate capacity the entire time.
Plant Factors vary greatly depending on the type of power plants and it is calculated according to the
following formula.
3.2 Running Plant Factor
The running plant factor of a generation unit is the ratio of the actual energy output of a generation
unit over a period of time to its potential output if it had operated at full nameplate capacity during
the period in which it has been operated.
Running Plant Factor shows the extent to which the generation units have been operated when they
are running out of their nominal capacities.
3.3 Equivalent Full Load Hours
This gives the number of hours during one year, during which the generator would have to run at
rated power in order to produce the energy delivered throughout a year.
Number of Equivalent Full Load Hours for a generator can be calculated using the formula given
below.
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Performance Measurements of Generation and Transmission Systems
3.4 Generation Unit Cost
Electricity generation is the process of generating electrical power from other sources of primary
energy. Electricity is by electromechanical generators, primarily driven by heat engines fueled by
chemical combustion or by other methods such as the kinetic energy of flowing water or wind. Apart
from that the other energy sources include solar photovoltaic and geothermal power.
However, there is a cost associated with power generation which mainly affects the electricity price.
This cost varies with the energy source incorporated with power generation and also could be
classified as energy cost and maintenance cost. The electricity generation cost of any power plant
can be used to find the cost of generation per one unit (1kWh) of electricity.
3.5 Auxiliary Consumption
Auxiliary system facility is a major part of a power generation facility and the auxiliary consumption
of a power plant depends on its configuration, age and related technical parameters. Purpose of an
auxiliary system is to supply power for its own electricity requirements. Auxiliary Consumption
Percentage is a very important performance indicator to evaluate the productive output from a
power plant.
Normally 0.5% - 2% of power generated is consumed for the auxiliary system in hydro plants while
the auxiliary consumption in fossil fuel power plants is 7% - 15% since there are different equipment
like feed pumps, cooling water pumps, air fan, etc.
3.6 Availability Factor
The evaluation of availability of a power plant is one of the most important tasks at any power
station which indicates the fraction of time that it is able to produce electricity over a certain period.
To analyze plant availability performance, generation unit outages should be scrutinized to identify
the causes of unplanned or forced energy losses and to reduce the planned energy losses. Reducing
outages increases the number of operating hours, therefore increases the plant availability factor.
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Performance Measurements of Generation and Transmission Systems
Availability Factor of a generation unit can be calculated using the formula given below.
4. Special Performance Indicators for Renewable Power Plants
Electricity generation using renewable energy sources is somewhat different from the other
conventional methods. These energy sources are freely available in the environment unlimitedly.
Therefore some special performance indicators can be defined in the technical, environmental, and
sociological point of views.
4.1 Wind Power Generation
 Specific Energy Production
The Specific Energy Production for a wind turbine is calculated using the formula and it can be
given in kWh/m2;
 Noise from Wind Turbines
Maximum noise of a wind turbine can be measured in dB from specifically defined distances
from wind turbine such as;

At the foot of the wind turbine

500m away from the foot of the wind turbine

At a standard distance (H + D/2) from the foot of the turbine
Where;
H = Height of the turbine tower
D = Diameter of the circle swept by the turbine blades
 Birds Fatalities
This is to measure the number of birds killed per wind turbine in the reported period
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Performance Measurements of Generation and Transmission Systems
4.2 Solar Power Generation
 Reference Yield
This measures the number of hours during which the solar radiation would be at reference
irradiance level in the reported period.
Where;
YR = Reference Yield
H = In-plane irradiation
GSTC = Reference irradiance under Standard Test Condition
4.3 Biomass Power Generation
 Fuel Moisture
This measures the moisture (H2O) content of the biomass as a fraction of the total weight.
 Ash Emission
Quantity of the ash emitted, with their compositions could be measured to assess the
environmental impact.
4.4 General
 Contribution to the Reduction of Green House Gas Emission
This measures the avoided CO2 emission (in metric tonnes per MW per year), compared to what
would have been emitted by a new power plant built in the region, given the same annual
production, using the fuel the most likely the future fuel choice, or by the plant most likely to be
displaced by the new renewable power plant.
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Performance Measurements of Generation and Transmission Systems
 Jobs Created by the Plant
This measures the number of jobs (direct/indirect) created by a power plant (in number of jobs
per MW) for different steps such as manufacture, installation, operation and maintenance.
 Industrial Safety Accident Rates
This measures the number of accidents for all utility personnel permanently assigned to the
plant, or fatalities per nominal man-hours worked.
5. Conclusion
The key objective of implementing the performance indicators is, measuring the performance to
compare them with other similar networks or systems and to assess the performance
improvements attained. The gathered performance measurements are also applicable for Sri
Lankan generation and transmission systems.
Whole transmission network in Sri Lanka is owned and operated by Ceylon Electricity Board and
most of the data required to compute the aforementioned transmission performance measures,
are already recorded by them. Data logging and recording facility should be implemented to
obtain the data which is unavailable (Specially power quality measures) to measure the
transmission performance.
Most of the data required to compute the generation performance measures of large hydro
power plants and fossil fuel fired power plants in Sri Lanka, are already available with the plant
operators. Most of the special performance indicators defined for the renewable plants are still
in the conceptual stage and therefore a proper mechanism should be implemented to record the
data required to compute these performance measures. Perhaps, Data logging and recording
facility may be required to be implemented to obtain the data which is unavailable to measure
the performance.
Once these performance measures are standardized and computed, they can be deployed not
only to monitor the quality and reliability of the electricity supply, but also to measure the
performance compared with benchmarks and to illustrate the historical trends.
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Performance Measurements of Generation and Transmission Systems
6. References

Regulation on Transmission Licensee Standards of Performance of Andhra Pradesh Electricity
Regulatory Commission

National Electricity Transmission System Performance Report of National Grid Company, UK

5th CEER Benchmarking Report on the Quality Of Electricity Supply-2011

Transmission Performance Standards Code - Electricity Regulatory Commission Of Jordann

System and Network Performance Indicators for the Electricity Generating Authority of
Thailand - Electricity Generating Authority of Thailand

Transmission Network Performance Report to Australian Energy Regulator – 2005

Report to The Gas & Electricity Markets Authority -GB Transmission System Performance
Report 2007 – 2008

LANDSNET Performance Report 2010 (Iceland)

Electricity Network Performance Report - New South Wales

Transmission System Performance Report for The Month Of January – March, 2011 (Bhutan
Power Corporation Limited System Coordinator Office )

Transmission System Performance Report for The Year 2008/09 - System Operator For
Northern Ireland Ltd

Transmission Performance Assessment by Edward A. Kram (P.E., Member, IEEE)

Eirgrid Transmission System Performance Report 2007 (Ireland)

SP Transmission Ltd - Transmission System Performance Report (Scotland)

Key Performance Indicators of a Transmission System by Omar H. Abdalla, Masoud AwladThani, Mohamed Al-Wardi, Khalfan Al-Qaidi, Saqar Al-Farsi, Ibrahim Al-Balushi, and Saeed AlMahdhoori (Oman Electricity Transmission Company)
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